MADRID - SPANISH security services cordoned off the US embassy in central Madrid on Wednesday after a suspicious envelope sparked what turned out to be a false security alert, police said.

'It's a false alert, the envelope contained nothing,' a police spokesman told AFP.

Security forces surrounded the building and blocked off nearby streets after an envelope was found containing a suspicious white powder, sparking fears of a chemical or biological attack.

Police said the powder turned out to be flour.

An embassy spokesman, William Ostick, said the envelope 'appeared to match the characteristics of letters received by other US embassies in Europe and government offices in the United States.

'We closed the entrance and the mail handling facility that had processed the letter, but the embassy was not closed,' he said.

The State Department in Washington confirmed the powder was innocuous.

'Apparently the embassy received an envelope with white powder. I believe the contents were found to be harmless,' State Department deputy spokesman Robert Wood told AFP. -- AFP